In motor vehicles, wheel brakes which can be activated hydraulically or electromechanically are provided for operating a vehicle brake system. As a rule, the wheel brakes which can be activated hydraulically are arranged on at least one of the front axles, and if appropriate, wheel brakes which can be activated electromechanically are arranged on at least one of the rear axles of the motor vehicle.
In vehicles with a hybrid drive, the drive can be provided either by means of an internal combustion engine or by means of an electric motor; in an electric vehicle the drive is provided exclusively by means of the electric motor. The electric motor is connected for this purpose to one or more axles of the motor vehicle and drives said axle or axles. In the operating state of what is referred to as the regenerative mode (also referred to as regenerative brakes, regeneration=recovery), the electric motor can be operated as a generator, for example during deceleration maneuvers or when traveling on downward gradients which require a negative drive torque, and the electric motor can therefore generate electrical energy without using fuel and in the process simultaneously spare the brakes. As a result, the overall consumption of energy of the motor vehicle can be reduced, the efficiency can be increased, the wear on the brakes minimized and the operation of the motor vehicle therefore made more economic. The potential of recovering braking energy is determined from the ratio of the regeneration torque to the overall braking torque and is also dependent on the possibility of distributing the regeneration torque between the front and rear axles.
As a rule, motor vehicles having a brake system which is configured for regenerative braking have for this purpose various types of brakes which are also referred to as brake actuators.
As a rule, in hybrid vehicles a pair of hydraulic friction brakes are used as conventional brakes for braking the front axle wheels and/or for the rear axle wheels, such as are known from conventional motor vehicles. Alternatively or additionally, a pair of friction brakes which can be activated electromechanically can preferably be used for braking the rear axle wheels. Usually at least some of the total amount of braking force which is necessary or desired by the driver is provided by the generator or the electric motor which is in the generator mode. The electrical energy which is acquired in the process is fed into or fed back into a storage medium such as, for example, an on-board battery and is used again for driving the motor vehicle by means of a suitable drive.
During the braking of a vehicle which has an electric motor whose generator mode is used to recover braking energy, a further braking torque, referred to as the regeneration torque, which is provided by the electric motor, is generated in addition to the braking torque of the conventional wheel brakes which can be activated hydraulically and electromechanically. This braking torque of the electric motor arises from the known generator effect in electric motors which act as a dynamo or a generator in mechanical drives without a supply of electric current, and generate electric current. In the process, an opposing force which counteracts the mechanical drive is produced and in the present case said force acts as a braking torque. The total braking force of the motor vehicle is therefore composed of the braking force of the wheel brakes which can be activated hydraulically, the braking force of the wheel brakes which can be activated electromechanically and the braking force of the electric motor which acts as a generator (regeneration torque, regenerative braking torque).
DE 103 19 663 A1 discloses a method for setting the pedal characteristic curve of a hybrid brake system with variable distribution of the braking force. In the previously known method for operating a brake system, which method comprises a hydraulic service brake system and an electric service brake system with wheel brakes to which brake pressure is applied when a brake pedal is activated, a control device controls a brake pressure modulator of the hydraulic service brake system and the electric service brake system in such a way that when there is a change in the distribution of the braking force between the hydraulic service brake system and the electric service brake system, the ratio of pedal force and/or pedal travel to the total braking torque of the vehicle remains essentially constant. An electric motor which can be operated as a generator is not provided.
In vehicles having a hybrid drive or an (exclusively) electric drive, the electric generator on one or more of the axles of the vehicle becomes active as soon as the driver takes his foot off the accelerator pedal or brakes. As a result, the regeneration torque which is generated by the regeneration and which leads to a reduction in the adhesion at the vehicle wheels takes effect, extending as far as loss of the adhesion. If this additional regeneration torque occurs in a boosted fashion at the rear axle, the rear axle is referred to as being overbraked. Overbraking causes the motor vehicle to lose its driving stability, with the result that the vehicle veers off or skids.
It can become critical in terms of vehicle movement dynamics if the distribution of the braking force shifts in the direction of the rear axle. In vehicles in which the generator acts exclusively/mainly on the rear axle, hard braking of the rear axle in order to achieve the highest possible recovery of energy involves the risk of overbraking of the rear axle. For this reason, an electronic vehicle movement dynamics control system for restoring possible instability caused by the driving concept is virtually indispensable.
Conventional strategies for controlling the generator torque permit unrestricted regeneration until the electronic braking-force distribution control system (EBD) engages owing to wheel instability or vehicle instability. In this case, the regeneration is frequently aborted and subsequently conventional braking is continued. A disadvantage of this strategy is that the critical situation which forces the EBD system to engage is caused by the regeneration strategy itself. As a result, the number of pump activities and valve activities increases significantly, which has to be taken into account in the configuration of the hydraulic system.